With reference to FIG. 1, a ducted fan gas turbine engine generally indicated at 10 has a principal and rotational axis X-X. The engine comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high-pressure compressor 14, combustion equipment 15, a high-pressure turbine 16, and intermediate pressure turbine 17, a low-pressure turbine 18 and a core engine exhaust nozzle 19. A nacelle 21 generally surrounds the engine 10 and defines the intake 11, a bypass duct 22 and a bypass exhaust nozzle 23.
The gas turbine engine 10 works in a conventional manner so that air entering the intake 11 is accelerated by the fan 12 to produce two air flows: a first air flow A into the intermediate pressure compressor 14 and a second air flow B which passes through the bypass duct 22 to provide propulsive thrust. The intermediate pressure compressor 13 compresses the air flow A directed into it before delivering that air to the high pressure compressor 14 where further compression takes place.
The compressed air exhausted from the high-pressure compressor 14 is directed into the combustion equipment 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines 16, 17, 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines respectively drive the high and intermediate pressure compressors 14, 13 and the fan 12 by suitable interconnecting shafts.
The combustion equipment 15 typically includes a combustor case, which may be manufactured from a nickel base superalloy, such as Waspaloy™. Such alloys provide high temperature capabilities. However, with the demand for ever increasing engine efficiencies, combustion temperatures are increasing to a point where conventional superalloys do not satisfy thermal capability and creep-life requirements.
Thus new materials with improved creep-life performance at higher temperatures are being adopted. One such material is the nickel base superalloy (hereafter designated as RR1000) described in U.S. Pat. No. 5,897,718.
RR1000 has excellent high temperature properties, but it is extremely difficult to machine and form/forge, and as a result, conventional machining may be economically or technically prohibitive. In particular, techniques used to manufacture Waspaloy combustor casings, such as ring roll forming and machining, are not suitable for RR1000. Components made from RR1000 are, therefore, typically formed via a powder metallurgy route.
For example, Hot Isostatic Pressing (HIP) is a near net shape technique which can be used to form RR1000 combustor casings. The HIP process uses an inner and outer metal (e.g. carbon steel) tool whose internal surfaces have conventionally machined features which are a negative form of the features required on the net shape formed component. The assembled tool is filled with RR1000 powder and the powder is compressed. The tool and powder form is then placed into a large HIP vessel that maintains a compressive force on the tooling and on to the powder within the tooling cavity. The temperatures and pressures applied during the HIP process are sufficiently high within the HIP vessel to diffusion bond the RR1000 powder particles together into a workpiece.
Unfortunately, however, elements from the tooling also diffuse into the outer layer of the RR1000 workpiece. This diffusion band has undesirable effects on material properties. In particular, the carbon from even an ultra low carbon steel tool can migrate into the RR1000 grain boundary interstitials, stressing the boundaries. Iron also diffuses into the powder alloy, producing undesirable effects. Crack initiation sites can result which could cause catastrophic part failure if unaddressed. However, as indicated above, conventional machining of the whole outer surface of the workpiece to remove the diffusion band would be extremely difficult due to the RR1000 material properties and time consuming, and if carried out would result in an unacceptably high part cost for the casing.